Some new designs of mobile devices—such as smart phones, tablet computers, and laptop computers—include two or more radio access technologies (“RATs”) that enable the devices to connect to two or more radio access networks, such as Long Term Evolution (LTE) networks and Global System for Mobile Communications (GSM) networks. Such mobile devices may include a single radio-frequency (RF) communication circuit or “RF resource” for providing access to separate networks via two or more RATs, each RAT associated with a network subscription or Subscriber Identity Module (SIM). For example, a first subscription (e.g., a GSM network subscription) may use a transceiver of the mobile device to transmit to a GSM base station at a first time, and may use the transceiver to transmit to an LTE base station via a second subscription (e.g., a LTE SIM) at a second time. Such mobile devices are often referred to as Dual-Subscription, Dual-Standby (DSDS) mobile devices. Conventional DSDS mobile devices may include a call manager module (“CM” in the drawings) or a multi-mode call manager that acts as a central coordinator and organizes all calls associated with various subscriptions, such as by managing call origination and termination. For example, the call manager may generate call IDs for incoming calls (which are referred to as mobile terminating or “MT” calls) or outgoing calls (which are referred to as mobile originating or “MO” calls) on a Universal Mobile Telecommunications System (UMTS)/GSM/Wideband Code Divisional Multiple Access (WCDMA) subscription and/or an LTE subscription. Therefore, the call manager may be aware of the status of various packet-switching (PS) calling. For example, based on information from an IP Multimedia Subsystem or IP Multimedia Core Network Subsystem (IMS) within a mobile device, the call manager may be aware of when a PS call has completed.
As there is only one RF chain for use by subscriptions in a DSDS mobile device, such devices must periodically tune its transceiver to another frequency in a “tune-away” process, momentarily switching from a first subscription to a second subscription in order to receive or monitor communications, such as pages. During such a tune-away, only the second subscription receives communications while the first subscription is temporarily unable to send or receive. The DSDS mobile device is configured to block tune-aways from subscriptions engaged in high-priority circuit-switch (CS) calls, such as voice calls or emergency calls/priority calls on a UMTS/GSM/WCDMA subscription. For example, a non-access stratum layer in a UMTS scenario may cause a lower layer access stratum to block tune-aways from an active CS voice call. The DSDS mobile device is also typically configured to enable tune-aways during packet-switch (PS) data calls without impeding the operations of the PS subscription.
However, subscriptions capable of PS communications may be capable of both voice and data packet exchanges. Accordingly, users of DSDS mobile devices may have suboptimal experiences conducting PS voice calls, such as voice over LTE (VoLTE) calls on an LTE subscription, as tune-aways from a packet-switching subscription to a circuit-switching subscription cause loss of packet data that degrades the experience and quality of the PS voice call.